NB.original +/(((0=3|/1+i.1000-1) +. (0=5|/1+i.1000-1)) * (1+i.1000-1))
NB.refactored
+/(*(3|])+.&(0=[)5|])1+i.1000-1
+/~.(3*i.>.1000%3),5*i.>.1000%5 NB.alternative
+/((0=3|n)+.0=5|n)#n=:i.1000 NB.alternative